Effects of flow interruption on transport and retention of iron oxide colloids in quartz sand

Abstract

Due to the complexity of flow conditions as well as solid matrix and colloid surface properties, solid knowledge about the transport of iron oxide colloids in soils remains scarce. In order to analyze the influence of flow conditions on iron oxide colloid transport and retention, breakthrough behavior of negatively charged, organic matter-coated goethite (OMCG) colloids in saturated quartz sand columns was investigated under continuous and stagnant flow conditions. Classic DLVO and extended DLVO (XDLVO) interaction energies including Lewis acid/base parameters were evaluated using measurements of sessile drop contact angles and zeta potentials of OMCG colloids and quartz. Results elucidated that under continuous flow conditions, OMCG colloids were highly mobile, which was in agreement with calculated unfavorable attachment conditions revealed by predictions of both DLVO approaches. In contrast, during intervals of flow interruption, significant amounts of OMCG colloids were retained in the solid matrix and could not be remobilized via re-establishment of flow. The magnitude of colloid retention increased with the duration of flow interruption; OMCG colloids were almost completely immobilized after 112h. Further experiments were conducted in order to determine possible colloid retention mechanisms. Results indicated that the major cause for retention during flow stagnation was OMCG colloid capture at locations with attractive DLVO/XDLVO interactions, promoted by fast gravitational settling of colloids onto the solid matrix. We compared breakthrough curves to model predictions, where we demonstrated that an attachment term with a stagnant fluid switch was required in the mass balance in order to reproduce the measurements. We conclude that high mobility of OMCG colloids and prediction of that transport behavior with the applied DLVO approaches were only valid under continuous flow conditions. Under more discontinuous hydraulic conditions relevant in natural soils, such as flow interruption, OMCG colloid transport behavior was modified significantly.